more operations;

--- a/src/Pure/System/process_result.scala	Mon Oct 03 18:54:59 2016 +0200
+++ b/src/Pure/System/process_result.scala	Mon Oct 03 20:09:50 2016 +0200
@@ -40,4 +40,7 @@
     Output.writeln(Library.trim_line(out), stdout = true)
     copy(out_lines = Nil, err_lines = Nil)
   }
+
+  def print_if(b: Boolean): Process_Result = if (b) print else this
+  def print_stdout_if(b: Boolean): Process_Result = if (b) print_stdout else this
 }